The present invention relates to a heat exchanger abnormality monitoring system that can monitor the presence of scale accretion to flow distribution devices and the outer and inner surfaces of heat exchange tubes, as well as other abnormalities in accordance with necessity, provided in a heat exchanger for feed water heaters and the like installed in feed water or condensate water systems of power generation plants.
In general, feed water or condensate water systems of power generation plants have heat exchangers, that is feed water heaters installed in order to heat feed water or condensate water by the steam discharged from steam turbines.
The following is a general description of such a feed water heater, with reference to the thermal power generation plant shown in FIG. 31.
The steam created by a boiler 1 is led to a high-pressure turbine via a steam stop valve and an incrementation-reduction valve 2, and after it has fulfilled its function as a drive source, is led to a low-pressure turbine 4 where it functions as a drive source once again, and is cooled by the cold water in a condenser 5 to become condensate. The high-pressure turbine 3 and the low-pressure turbine 4 that are driven by the steam drive load of the generators 15, and the like.
In addition, the condensate stored in the condenser 5 is pressure fed by a circulation pump 6 and is supplied to a deaerator 8 via a feed water heater 7. The condensate (which after this is termed `feed water`) that is supplied to the deaerator 8 is pressure fed by the feed pump 10 and is supplied to the boiler 1 via a feed water heater, and is heated in the boiler once again to become steam.
Moreover, the extracted steam that is led via the steam extraction pipes 11,12 from the intermediate stages of the high-pressure turbine 3 and the low-pressure turbine 4 is led to the feed water heaters 7,9 described above and the condensate and the feed water are heated by the extracted steam with condensate and feed water of a higher temperature being supplied to the boiler 1.
Moreover, after the feed water and the condensate water have been heated, the extracted steam is drained and the drain inside the feed water heater 9 is led to the deaerator 8 via the drain pipe 13, and the drain inside the feed water heater 7 is led to the condenser 5 via the drain pipe 14 and flows into the condensate.
FIG. 32 shows one example of a feed water heater used in a thermal power generation plant having such a configuration. Moreover, the previous description was given in terms of heating the condensate and the feed water but for either of the feed water heaters, the process fluid in the power generation plant is generally water, and this process fluid and the extracted steam for heating only have different pressures and temperatures and so in FIG. 32, the description will be given in terms of the example of a feed water heater 9 for feed water.
The feed water heater 9 is largely configured from the two portions of a water chamber portion 21 and a feed water heater unit 22.
The water chamber portion 21 is divided by a water chamber partition plate 30 into an inlet-side water chamber 25 and an outlet-side water chamber 26. These inlet-side water chamber 25 and outlet-side water chamber 26 are linked by the many heat exchange tubes 31 that are disposed inside the feed water heater unit 22. In order to distribute the flow of feed water to the inlet-side water chamber 25, the feed water inlet portion 23 has mounted to it a flow distribution device 24 provided with small holes for the many feed water supply paths. In addition, the water chamber portion 21 has a manhole 32 mounted to it in order to allow inspection and maintenance of the inside of the water chamber drain cooling zone when the power generation plant is stopped.
In such a feed water heater 9, the feed water that is pressure fed by the feed water pump 10 passes from the feed water inlet portion 23 of the water chamber portion 21 and then through the flow distribution device 24 and flows into the inlet-side water chamber 25. The feed water that flows into the inlet-side water chamber 25 flows into the many heat exchange tubes 31 that are provided inside the feed water heater unit 22 and flows out to the outlet side water chamber 26, through the feed water outlet portion 27 and is fed to the boiler 1.
The vicinity of the feed water heater unit 22 through which the extraction steam passes is known as the desuperheating zone 33, and the vicinity where the drain flows from the feed water heater is known as the drain cooling zone 35, and the other portions are known as the condensing zone 34. However, the extracted steam flows from the extracted steam inlet portion 28 to the desuperheating zone 33 and heat exchange with the feed water that flows inside the heat exchange tubes 31 cools it so that it becomes saturated steam which then condenses in the condensing zone 34 to become high temperature water which is drained.
After this, this drain is further cooled in the drain cooling zone 35 by heat exchange with the feed water that flows inside the heat exchange tubes 31, becomes low-temperature drain and flows from the drain outlet portion 29 of the feed water heater unit 22 and flows into the deaerator 8 via the drain pipe 13. On the other hand, the feed water inside the heat exchange tubes 31 flows from the inlet-side water chamber 25 to the outlet-side water chamber 26 and is gradually superheated along the way.
Moreover, depending upon the power generation plant, there are instances where a plural number of feed water heaters are disposed in series and respectively for feed water and for condensate water but in many cases such as these, the drain that flows from the drain outlet portion 29 of the feed water heater that is closest to the side of the boiler, is supplied to the drain inlet portion 36 provided to the condensing zone in the vicinity of the drain cooling zone of the feed water heater closest to the condenser. In such a feed water heater, the drain that is the result of cooling of the extracted steam that flows to that feed water heater after the vicinity of the drain cooling zone, and the feed water that flows inside the heat exchange tubes by the drain that flows in from the feed water heater on the side of the boiler, are heated.
Also, there are many cases where general carbon steel pipes are used as the flow distribution device 24 and the heat exchange tubes 31 and the like. When such a feed water heater is operated for extended periods of time, the iron component in the feed water accretes to the flow distribution devices and to the inner surfaces of the carbon-steel tubes that are in contact with the feed water, to result in the generation of a scale membrane of ferrous oxide and which is known as magnetite.
This magnetite scale membrane adheres strongly and is formed thinly over the entire inner surface of the steel pipes and is effective in protecting and preventing the corrosion of the steel tubes. However, when this scale membrane accretes thickly on the inner surfaces of the many small holes that are provided to the flow distribution device 24 and the inner surfaces of the steel tubes, that is, in the feed water flow path, the feed water flow path area is reduced and so there is a relatively larger pressure loss for the feed water in the flow distribution device 24 and the steel tubes and the like when compared to that at the same flow rate when the scale membrane is relatively thin. More specifically, there is an increase in the differential pressure of the feed water pressure on the outlet side and the inlet side of the flow distribution device 24 and the differential pressure of the feed water pressure on the outlet side and the inlet side of the steel tubes. When the thickness of this scale membrane becomes excessive, the differential pressure on the outlet of the flow distribution device 24 becomes excessive to give rise to the fear of destruction of the flow distribution device 24, and if the differential pressure of the outlet of the steel tubes, that is, the inlet-side water chamber 25 and the outlet-side water chamber 26 becomes large, there is the danger of the destruction of the water chamber partition plate 30.
In addition, the feed water flow rate to the boiler 1 is determined by the increase or decrease in the turbine load of the power generators and the like but when the feed water flow rate corresponding to the load of the generators is pressure fed to the boiler 1 by the feed water pump 10 and via the feed water heater 9, there is a feed water pressure loss in the steel tubes and the flow distribution devices which is greater when there is a thick scale membrane than when there is a thin scale membrane and the discharge pressure of the same pump must be increased by raising the speed of the feed water pump, to result in the problem of an increase in the pump load.
As has been described above, heat exchange between the drain and the extracted steam and the feed water is performed via the tube walls in the heat exchange tubes 31 but there is the problem that the thicker the accretion of a scale membrane on the inner surfaces of the heat exchange tubes, the lower the heat exchange efficiency between the drain and the extracted steam and the feed water in the feed water heater.
Furthermore, in feed water heaters for which the temperature of the feed water is relatively low, the magnetite scale membrane that is formed is relatively weak and so it is easy to flake and peel off. On the other hand, for feed water heaters that operate with relatively high-temperature feed water at temperatures in the two hundreds (.degree.C.), the magnetite scale membrane is relatively firmly attached. However, even in cases such as these, mechanical shock and the like caused by rapid changes of the flow speed, or turbulence of the feed water flowing inside the feed water heater, or thermal shock caused by differences in the coefficients of thermal expansion and the coefficient of heat transmission between the scale membrane and the flow distribution devices and the steel tubes and resulting from changes in the load and from stopping and starting of the power generation plant can generate local peeling or flaking of the magnetite scale membrane and the scale membrane that is peeled off moves to the downstream side by the flow of the feed water and collects in the feed water flow path portion to cause the problem of the feed water pressure loss increasing even further. Not only this, the local flaking of the scale membrane causes the problem of the generation of channel or hole corrosion in the steel tubes.
In this manner, the magnetite scale membrane is effective while there is the generation of a thin layer of scale across the entire inner surface of the steel tubes but when this thickness is excessive, it is necessary to perform work to remove the scale membrane since various types of problems as described above occur.
This work to remove the scale membrane if it has formed in the flow distribution devices, can be performed by opening the manhole 32 of the water chamber portion 21 while the power generation plant is stopped, and by then removing and performing cleaning work for the flow distribution device 24. This is comparatively simple work but for the scale membrane that has generated in the inner surface of the steel tubes, not only does the manhole 32 of the water chamber portion 21 have to be opened while the power generation plant is stopped, but it is also necessary to remove the water chamber partition plate 30 and to insert a high pressure water flow into the steel tubes and perform cleaning, so that it is necessary to reattach the water chamber partition plate 30 by welding or the like and this of necessity involves much troublesome related work.
It is therefore extremely effective to monitor the degree to which the scale membrane is generating, particularly if the scale membrane is excessive.
However, in the past, there has been no effective apparatus that can monitor the status of generation of the magnetite scale membrane that attaches to the steel tube and the flow distribution devices, while the power generation plant is operating.
Depending on the power generation plant, the differential pressure of the feed water pressure in the feed water outlet portion 27 and the feed water inlet portion 23 of the feed water heater is periodically measured by a differential pressure gauge and when that absolute value has increased to greater than a value greater than the previously measured value, work to remove the scale membrane is performed since the thickness of the scale membrane has increased to become excessive.
However, when there is monitoring using a differential pressure gauge, the measurement is difficult and cannot be considered to be reliable. More specifically, in the case of a specific example of a feed water heater for a power generation plant, the feed water inlet portion pressure of the feed water heater installed between a feed water pump and a boiler is approximately 300 Kgf/cm.sup.2 and the feed water outlet portion pressure is approximately 298.5 Kgf/cm.sup.2 when there is only a thin (acceptable) scale membrane (while the differential pressure gauge shows approximately 1.5 Kgf/cm.sup.2) and is approximately 297 Kgf/cm.sup.2 when there is an excessively thick generation of scale membrane. This means that the pressure of the feed water pressure that has to be measured is only in the small range of 0.5 to 1.0%. Also, the respective feed water pressures at the feed water inlet portion 23 and the feed water outlet portion 27 are always fluctuating minutely (in what is commonly termed "pulsations") and despite the fact that the feed water pressure is normally derived as:
feed water inlet portion pressure&gt;feed water outlet portion pressure. PA1 feed water inlet portion pressure&lt;feed water outlet portion pressure.
There are also occasions when the differential pressure of the feed water pressure indicated by differential pressure measurement is a negative value shown by:
Also, depending on the increase or decrease of the load at the power generation plant (in other words, to increase or decrease the feed water rate) even if the thickness of the scale membrane is the same, this differential pressure value changes greatly and so if a comparison of the differential pressure is not performed for when the feed water flow rate is the same value, it is not possible to make a judgment whether the thickness of the scale membrane is excessive or not. Accordingly, the differential pressure had to be measured and compared to the same timing as the load (feed water flow rate) value for when there are differential pressure measurements for when the past scale membrane was thin (acceptable).
In addition, because of the structure of the feed water heater, the water chamber partition plate generally has a lesser strength with respect to differential pressure than does the flow distribution devices and is easily broken even by small differential pressures so that, if possible, the differential pressure should be monitored for each of the flow distribution devices and the steel tubes. But performing this means that the differential pressure of the feed water pressure in the inlet side water chamber and the feed water output portion or the output side water chamber, and the differential pressure of the feed water pressure in the feed water inlet portion and the inlet side water chamber has to be monitored. More specifically, in addition to the feed water outlet portion and the feed water inlet portion, the feed water pressure at the inlet side water chamber also has to be measured. But in general, the structure of the intake side water chamber portion is complex, and so it is not easy to install piping for pressure measurement at this portion. Furthermore, since the internal structure in the inlet side water chamber portion and the outlet side water chamber portion is also complex, there is the tendency for disturbances to occur inside, with pulsations becoming larger, the larger these disturbances are. In addition, the differential pressure of the feed water pressure that has to be measured is also smaller than the differential pressure of the feed water outlet portion pressure and the feed water inlet portion pressure as has been described above, and this means that the measurement results that are obtained have even less reliability than those of the case described above and it is not possible to determine which portions have an excessive thickness of scale membrane generation.
On the other hand, depending upon the power generation plant, the degree of heat exchange between the drain and the extracted steam of the feed water heater is monitored via a steel tube. When this degree of heat exchange drops, it is judged that there is the generation of an excessive thickness of scale membrane inside the steel tubes and work to remove this scale membrane has been performed.
However, when the degree of heat exchange is monitored, the difference between the feed water temperature at the feed water outlet portion and the extracted steam saturation temperature of the feed water heater, and the difference between the feed water temperature of the feed water outlet portion, and the drain temperature of the feed water heater are generally measured, but when the temperature difference between these two, when there is the generation of a thick scale membrane, changes by approximately 3.degree. C. when compared when there is only the generation of a thin scale membrane on the inner surfaces of the tubes. In addition, in a power generation plant, a thermocouple or a temperature measuring resistor is generally used in the temperature detector to measure the temperature of these portions. But the measurement method used in these temperature detectors is such that a measurement error due to the amount of time that has elapsed since installation changes by several degrees C. and so this method has been regarded as unreliable.
Moreover, the description so far has been for only the generation of scale membrane in the feed water side of the feed water heater, but in reality, a small amount of foreign matter included in the extracted steam attaches to the outer surfaces of the steel tubes when the power generation plant has been in operation for an extended period of time and as a result, the heat exchange performance drops (so that the difference between the feed water temperature at the feed water outlet portion and the extracted steam saturation temperature of the feed water heater, and the difference between the feed water temperature of the feed water outlet portion, and the drain temperature of the feed water heater outlet changes by about several degrees when compared to the normal situation).
In this manner, when there is the adhesion of foreign matter to the outer surfaces of the steel tubes, this differential pressure causes the partial destruction of the feed water heater but when this amount becomes excessive, the degree of heat exchange with the feed water heater drops so that it is not desirable to remove the foreign matter.
However, in cases such as these, it is unclear whether the drop in the degree of heat exchange is due to the accretion of scale membrane on the outer surface or the inner surface of the steel tube. So, when opening the manhole of the water chamber portion is performed as part of the work of removing the scale membrane on the inner surface of the steel tubes and the flow distribution devices, there are occasions when it is necessary to remove foreign matter on the extracted steam side but not on the feed water side. In addition, in the same manner as the method of using differential pressure measurements, even if the thickness of the scale membrane is the same, increases and decreases in the load at the power generation plant (that is, increases and decreases the feed water flow) cause the degree of heat exchange to change greatly. So, comparison of the degree of heat exchange must be performed when the load is the same value (feed water flow). Accordingly, it is necessary to measure and compare the degree of heat exchange at the same timing as the load value (feed water flow), at the time of measurement of the degree of heat exchange when there was a thin scale membrane in the past.
In addition, for as long as there is no generation of scale membrane on the outer or inner surfaces of the steel tube, this method has the disadvantage that there is no change in the degree of heat exchange no matter what the degree of excessive thickness of scale is with respect to the flow distribution devices. It is also not possible to determine the position where there is an excessive thickness of scale membrane.
Therefore, there are many cases where such measurement is not performed and where the power generation plant is operated for a predetermined period and then removal of the scale membrane is periodically performed.
However, in these cases, the manhole of the water chamber portion is opened and the flow distribution devices are removed, and the water chamber partition is removed and high-pressure water or the like is introduced into the steel tubes and cleaning is performed so that in some cases there is the inconvenience of finally knowing that there is no generation of an excessive thickness of scale membrane.
However, the major parts where there is scale accretion in and around the feed water heater are the inner and outer surfaces of the heat exchange tubes, the flow distribution devices and the drain level adjustment valve of the feed water heater.
The drain level adjustment valve of the feed water heater is installed in the drain pipe connected to the feed water heater drain outlet portion for the purpose of controlling the drain water level of the drain cooling zone of the feed water heater to a constant value, with the drain level of the drain cooling zone being detected, with control of this water level being performed to a predetermined value by opening and close control performed by the receiving of output signals of the drain water level adjustment gauge of the feed water heater, and the drain flow rate that flows from the feed water heater being controlled so that as a result, the water level of the drain that collects in the drain cooling zone inside the feed water heater is controlled at a constant level.
There are occasions where there is the accretion of a scale membrane to this drain level adjustment valve and the following problems occur when this accretion is excessive.
(1) When there is scale membrane accretion in the drain flow path portion and the drain flow path area is reduced, then even if the degree of opening of the drain water level adjustment valve is the same, then there is a reduced drain flow when compared to no scale accretion on the drain.
(2) When there is scale membrane accretion the drain flow path portion, there is a change in the flow characteristics of the drain water level adjustment valve, and control deteriorates.
(3) When there is an accretion of scale to an excessive thickness and the clearance of each of the portions of the drain water level adjustment valve is reduced, then the motion of this adjustment valve deteriorates and sticks on occasions.
When there is formation of a scale membrane having excessive thickness on the drain water level adjustment valve, it is desirable that this be detected as early as possible. If necessary, the drain water level adjustment valve is disassembled, and the accreted scale membrane removed.
However, with conventional technology, it has not been possible to detect the accretion of a scale membrane to the drain adjustment valve, while the power generation plant is still operating.
Therefore, when it is not possible to complete the scale removal work during the period of the periodic inspection or when there must be additional scale removal work for the drain water level adjustment valve once it is known that there is an excessive thickness of scale membrane accreted after the disassembly of the drain water level adjustment valve when there is the periodic inspection when the power generation plant is not operating, the removal of the scale is performed at the next periodic inspection. Until then, the plant is operated with the scale membrane present. There are many occasions when such operation of the plant cannot be avoided and this results in changes in the periodic inspection processes, and the expenses involved.
The problems that can occur in feed water apparatus are not only an excessive thickness of scale membrane attached to each of the portions of the feed water heater as has been described above, but also leaks in the tubes for heat exchange, problems of destruction of the water chamber partition plate that partitions the outlet side water chamber and the inlet side water chamber for the feed water, and problems of extracted steam being taken into the drain cooling zone or the short path of the drain due to plate destruction around the drain cooling zone of the feed water heater.
The problem of the generation of a leak in the tubes for heat exchange of the feed water heater is, more specifically, the generation of pinholes in one portion of the tubes of the heat exchanger or the problems of high pressure feed water from the connections between the heat exchanger tubes and the materials configuring the feed water heater, leaking to inside the low-pressure feed water heater unit, that is, the side of the drain or the extracted steam for heating. In cases such as these, the leak amount may be only small when the leak is first discovered but since the water is high pressure, the leak place enlarges in a relatively short time. Moreover, this leaked feed water becomes the same as the drain and is extracted from the feed water heater drain outlet portion via the drain water level adjustment valve but along with an increase in the leaking feed water flow, there is an increase in the drain amount that must be extracted from the feed water heater and so there is no alternative but to increase the degree of opening of the drain water level adjustment valve. In this case, when there is an increase in the leak feed water amount to a degree which is greater than of the drain amount that can be extracted when the drain water level adjustment valve is fully open, the drain from that feed water heater cannot be extracted and so the drain inside the feed water heater becomes full. Ultimately, the drain flows from the extracted steam inlet into either the high-pressure turbine or the low-pressure turbine via the extracted steam tubes.
Conditions such as this are generally known as `water induction` and when water induction generates in the low-pressure turbine or the high-pressure turbine that is driven by high-temperature steam, the materials of the high-pressure or the low pressure turbine that is at high temperature are quickly cooled by the relatively low-temperature drain and so there is the generation of cracking due to thermal stress and the consequent danger of a large-scale failure.
Accordingly, leaks in the heat exchange tubes must be detected at as early a stage as possible and the operation of the feed water heater stopped quickly and repairs performed to the connections between the heat exchange tubes and the materials of the water chamber, or the inlet portion and the outlet portions of heat exchange tubes in which there are pinholes or blocked passages and repairs must be performed so that there is no further leakage of feed water and then the feed water heater can be operated once again.
However, the detection of whether or not there is a leak in the turbines of the heat exchanger is conventionally performed by a judgement on the basis of an inspector listening by ear for the sound of a leak when there is the leak of feed water at high pressure. But feed water heaters normally have the sound of feed water flowing in and flowing out, and the sound of the flowing of extracted steam for heating. Also, these sounds change in quality along with changes in the load of the power generation plant. In addition to these, there are also cases where noise from many types of power generation plant equipment around the feed water heaters is also transmitted and it is difficult and requires much experience to distinguish these sounds from the specific sound that leaking water makes.
Therefore, instead of making a judgement on the basis of listening by a trained ear, an acoustic sensor such as an AE (acoustic emission) sensor or a acceleration type of acoustic sensor is used to detect the sound that is transmitted to the feed water heater, and this detected sound has signal processing such as frequency analysis of the detected sound performed for it so that only the sounds that are thought to be the sound of leaking feed water are extracted and detected from the sounds transmitted inside the feed water heater.
However, the nature of the sound of leaking feed water changes according to whether it is a leak from a hole or the type of opening, and according to whether it is a leak in the connectors between the water chamber, a leak in the superheating zone or a leak in the condenser drain cooling zone. Also, transmitted sounds other than the sounds of leaks, such as the sound of the flowing in of feed water or extracted steam, also change in complex ways, depending upon the load conditions of the power generation plant. Even if an apparatus such as has been described above is used, it is still difficult to reliably detect the appropriate sounds. In addition, it is still not possible to determine the places where the leaks are occurring.
In addition, there is no method available where it is possible to detect, while the power generation plant is operating, the short path of the drain or the inlet of extracted steam to the drain cooling zone due to the destruction of the water chamber partition plate of the feed water or destruction of the drain cooling zone enclosing plate.
Therefore, when there is an abnormal value indicated for the temperature of any of the portions when a monitor for the monitoring of the drain outlet temperature, the outlet/inlet feed water temperature of the feed water heater is exhibited, the trouble spot has been detected by the inspection of each portion by trial and error. In addition, there is the method of judgment trouble by the monitoring of the heat exchange performance of the feed water heater but when there is only monitoring of the heat exchange performance such as when for example, there is the destruction of the water chamber partition plate, it is not possible to discriminate between when there is the accretion of scale on the inner surface of the heat exchange tubes.
Still furthermore, there are also occasions when, depending upon the power generation plant, the discharge side of the feed water pump 10 branches into two or three branches (a plural number) and there are two or three (a plural number) feed water heaters installed in parallel, those output sides are again recombined in a pipe system that has a pressure feed to the boiler 1. With piping systems such as these, if the structures, performances and the like of the feed water heaters installed in parallel are exactly the same and if the flow path resistances are also exactly the same when there is the flow of fluid in the piping system, then the feed water amount that flows to the feed water heaters installed in parallel becomes exactly the same value (1/2 or 1/3 of the total feed water flow). Accordingly, the extracted steam amounts and the drain amounts that flow out to each of the feed water heaters also become exactly the same value.
However, when there is the accretion of scale membrane in amounts that differ completely for the flow distribution devices or the heat exchange tubes, with respect to the feed water heaters that are installed in parallel, differences occur in the flow path resistances with respect to the feed water for the respective feed water heaters and the feed water flows flowing into the respective feed water heaters are no longer the same. When this occurs, for example, if the heat exchange performances of the feed water heaters is the same, there are differences in the feed water outlet temperatures and the drain outlet temperatures and so it becomes difficult to determine the presence of abnormalities in the feed water heaters, the presence of scale accretion to the drain water level adjustment valve, or the flow distribution devices, or the inner or outer surface of the tubes for the heat exchanger.